Acoustic pulsations generate in the combustion chamber of gas turbines due to combustion instabilities. These pulsations can propagate in the fuel distribution system (FDS) and can be a source of equivalence ratio and hence heat release fluctuations that in turn enforce combustion instabilities in the combustor. Another source of pulsations in the FDS is flow-induced pulsations in the fuel line. This fuel feed instability mechanism can give rise to combustion instabilities in the combustion chamber. Pulsations in the FDS have been observed during combustor tests to reach high amplitudes. These high amplitude pulsations generate structural vibrations of the FDS that can be detrimental for structural integrity. Combustion instabilities are detrimental for the performances of combustion systems. They decrease the life-time of the combustor hardware and can therefore have a negative effect on gas turbine emissions. Therefore it has been appreciated that a mitigation measure is needed to damp pulsations in the FDS.
In existing technology, the fuel line pressure drop is increased to acoustically decouple the pulsations between the combustion chamber and the fuel distribution system. This increase poses the problem that higher pressure is required to drive the fuel in the engine. Furthermore, increased fuel pressure requirements can increase the cost of the engine, as a dedicated fuel compression system may be needed to obtain sufficient fuel pressure. In addition, this solution only counteracts the coupling between fuel line and combustion chamber but does not have any effect on damping any pulsations that are generated inside the fuel line (fuel feed instability mechanism). Other possibilities to decouple the pulsations between the combustion chamber and the fuel line are the use of gas flow restrictors and dynamically balancing the fuel nozzle. These solutions do provide decoupling of the pulsations between the combustion chamber and the fuel line but do not have an effect on damping the pulsations that are generated inside the fuel line.
Existing solutions that can be used to reduce the coupling between the combustion chamber and the fuel line and that can also reduce pulsations generated inside the fuel line use Helmholtz dampers or expansion chamber dampers. Expansion chamber solutions provide narrow-band acoustic absorption and are reactive dampers. A reactive damper provides acoustic transmission loss (TL) by reflecting back the incoming waves, reducing to a minimum the outgoing waves. The drawback of reactive dampers is that high pulsations can be generated in the fuel line where the incoming waves travel, due to the reflective characteristics of the damper. These high pulsations can give rise to high structural vibrations that are detrimental for the structural integrity.
It has been appreciated that improvements could be made to alleviate the above-mentioned problems.